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la
lb
lower in energy than those of type la.* Triplets l b can account for the results only if the nonplanar distortion is rapidly transferred from one end of the molecule to the other. 2 , 9 Acknowledgment. This research was supported in part by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by National Science Foundation Grant GP-7941.
We report now two examples of this type of asymmetric selection which are noteworthy not only because they serve as illustrations of the principle stated above but also because they provide rigorous bases for assignments of relative stereochemistry to previously unassigned stereomers. The diastereomeric forms (erythro and threo) of 1,2-diferrocenyl-1,2-diphenylethane3 occur among the products of Clemmensen reduction of benzoylferro~ e n e . While ~ these stereomers have been separated5 (mp3 218-220" and 276-278 "), no configurational assignments have been made. In the present work it has been shown that the lower and higher melting isomers are the erythro (1) and threo (2) forms, respectively. Treatment of (f)-ferrocenylphenylcarbinol (3) in ($)-(S)-l-methoxy-2-methylbutane (4) ( [ c r I z 1 ~0,032'; lit.6 [a]lsD 0.34") with zinc and hydrochloric acid gave the lower melting diastereomer (mp 218-222") in 45 % yield and the higher melting form (mp 276-278') in 31 % yield. Of the two purified products, only the higher melting isomer was optically active ( [ c r ] 2 z 5 4 6 26.1 i 1.7" (c 0.176, benzene)), showing it, therefore, to be the threo form 2.
(8) R. Hoffmann, Tetrahedron, 22,521 (1966). The calculations were for 1,3-butadiene and no distinction between triplets and singlets was made, (9) NOTEADDEDIN PROOF. We have discussed our results with Professor G. S. Hammond and Dr. B. M. Monroe. Since, it appears, the main difference in operation is our use of freshly distilled 2,4hexadienes and since we have some indication that undistilled diene isomerizes somewhat faster than distilled diene, we agree that it is probable that distillation eliminated some photoactive impurity. (10) (a) Petroleum Research Fund Fellow. (b) Petroleum Research Fund Scholar.
Jack Saltiel, Lewis Metts,loe Mark WrightonlOb Department of Chemistry, The Florida State University Tallahassee, Florida 32306 Received July 23, 1969
Differentiation of Relative Configurations in the erythro and threo Forms of 1,2-Diferrocenyl-1,2-diphenylethaneand 1,2-Diferrocenyl-1,2-diphenyl-1,2-ethanediol
Sir : A little explored area of stereoselectivity lies in the use of optically active solvents in reaction processes. In principle, one might expect to observe enantiomeric enrichment (asymmetric selection I) in the chiral products of such processes if the diastereomeric transition states, resulting from interaction of a chiral solvent with the transition states leading to enantiomeric products, give rise to a AAG* large enough to provide a significant rate difference in the formation of each enantiomer. (1) S. I. Goldberg and M. S . Sahli, J . Org. Chem., 32, 2059 (1967). (2) Optically active solvents, or solvent additives, that have been used successfully are: (+)-bornyldimethylamine (M. Betti and E. Lucchi, Boll. Sci. Fac. Chim. Ind. Bologna, 2 (1940)); (+)-2,3-dimethoxybutane (H. L. Cohen and G . F. Wright, J . Org. Chem., 18, 432 (1953); A. G. Brook, H. L. Cohen, and G . F. Wright, ibid., 18, 447 (1953); K. R. Bharucha, H. L. Cohen, and G. F. Wright, ibid., 19, 1097 (1954); N. Allentoff and G . F. Wright, Can. J . Chem., 35, 900 (1957); N. Allentoff and G. F. Wright, J . O r g . Chem.,22, l(1957)); (+)-mannitolhexamethyl ether and (-)-arabitol pentamethyl ether (N. Allentoff and G . F. Wright, ibid., 22, 1 (1957)). For an account of others see J. Mathieu and J. Weill-Raynal, Bull. Soc. Chim. Fr., 1211 (1968). For a n account of some early unsuccessful experiments in this area see P. D. Ritchie, "Asymmetric Synthesis and Asymmetric Induction," Oxford University Press, 1933, p 78 ff.
-6
7
8 *
In another experiment where a higher molar ratio of the optically active solvent 4 to 3 was used (51 as compared to 32 in the initial experiment), the threo isomer 2 was produced in lower material yield but in higher optical yield ([0!]~~546 57.3 f 5.5" (c 0.183, benzene)), indicating perhaps the presence of an interesting dependence of optical yield on the concentration of the chiral solvent. These results have also allowed assignment of stereochemistry to 1,2-diferrocenyl-1,2-diphenylethene,another product of the Clemmensen reduction of benzoylferrocene4 which is produced in that reaction in only one of its two diastereomeric forms. Hydroboration of the olefin, followed by treatment with propionic acid, (3) A. Berger, W. E. McEwen, and J. Kleinberg, J . Amer. Chem. Soc., 83, 2274 (1961). (4) S . I. Goldberg and M. L. McGregor, J . Org. Chem., 33, 2568 (1968). ( 5 ) S. I . Goldberg and W. D. Bailey, unpublished. (6) H. G. Rule, E. B. Smith, and J. Harrower, J . Chem. Soc., 376 (1933).
Communications to the Editor
5686
gave only the threo hydrocarbon 2. Since this transformation is well documented' as an over-all syn addition, it follows that the tetrasubstituted olefin (mp 278279 ") must possess the E or trans configuration (5). While 1,2-diferrocenyl-l,2-diphenyl-l,2-ethanediol has been suggested as a possible intermediate in the Clemmensen reduction of benzoylferrocene (6),8 its isolation from that reaction has recently been accomplished in this laboratory.5 As expected the pinacol was produced in two diastereomeric forms (mp 202-204 and 207-209 "). In another example of asymmetric selection brought about by the presence of an optically active solvent, it was found that only the higher melting pinacol is optically active, and therefore must be the threo isomer 8. Treatment of a solution of benzoylferrocene (6) in (+)-(S)-l-methoxy-2-methylbutane (4) with zinc dust and aqueous hydrochloric acid gave the two diastereomeric pinacols in good yield. Separation was achieved by fractional crystallization: the less soluble isomer (mp 207-209") was optically active (threo-8, [a]21j46 9.2 i 1.0" (c 1.0,benzene)), while no optical activity could be detected in the more soluble isomer (mp 202-204', erythro-(7)). Thus, the relative stereochemical features of all the relevant compounds involved in the Clemmensen reduction of benzoylferrocene have been rigorously elucidated by novel r n e a n ~ . ~
32.2 kcal/mol and log A = 12.8. In decalin (cis: trans = 2 : I ) at 130", I racemizes with k,,, = 1.44 X 10-5 sec-I, compared to k,,, = 3.34 X 10-5 sec-1 for the racemization of 2 under identical conditions.' That an allylic rearrangement is not implicated in the racemization of 1 was demonstrated by the observation that the pmr spectrum of the labeled compound,8 l-cu-d2, suffered no perceptible change when solutions in C6D6 were heated6 for prolonged periods of time at temperatures up to 150". At 175",a slow exchange of protons at the a-and y-allylic positions (7 7.6-7.8and 4.95-5.36, respectively) was noted, accompanied by slight decomposition. The half-life for exchange at 175",ea. 50 hr, is about 300 times the half-life for racemization at the same temperature, ca. 10 min. The allylic rearrangement, whatever its mechanism, is therefore too slow9 to account for the racemization, which must occur by pyramidal inversion. (7) This value is in good agreement with the rate constant, krac = 3.78 X 10-5 sec-1, previously reported for the racemization of 2 in decalin a t 130" (L. Horner and H. Winkler, Tetrahedron Lett., 461 ( 1964)). (8) Reaction of methylphenylbromophosphine with allyl-r-dz alcohol, followed by rearrangements of the allyl-7-dz methylphenylphosphinite (i) a t loo", gave a phosphine oxide (ii), reduction of which with hexachlorodisilane4 afforded the specifically deuterated phosphine (1-a-dz).
0 CBHs(CHs)POCHzCH=CD2
(7) H. C. Brown, "Hydroboration," W. A. Benjamin, Inc., New York, N. Y., 1962, p 128 ff. (8) M . D. Rausch and D. L. Admas, J . Org. Chem., 32, 4144 (1967). (9) R . W. Murry, R . D. Youssefyeh, and P. R. Story ( J . Amer. Chem. SOC., 88, 3655 (1966)) have assigned the relative configurations to cisand fmns-2,5-dimethylhexene-3 ozonides by means of kinetic resolution, a process similar in principle to that used in the present work.
Stanley I. Goldberg, William D. Bailey
l-a-d, (9) I n a related observation, M. P. Savage and S. Trippett, J. Chem. Soc., C, 1998 (1967), found that in the absence of catalysts diphenyl(1 -phenylallyl)phosphine does not rearrange to cinnamyldiphenylphosphine on heating a t 220" for 15 hr. (10) National Science Foundation Trainee, 1968-1969.
Raymond D. Baechler, William B. Farnharn,'O Kurt Mislow Department of Chemistry, Princeton Unicersity Princeton, New Jersey 08540 Received July 22, 1969
Stabilization via Carbon-Carbon Hyperconjugation
Sir : It has been suggested2 that the facile racemization2 of allylmethylphenylphosphine (1) relative to methylphenyl-n-propylphosphine (2) might be attributed to an intramolecular allylic rearrangement. 3 We wish to report that the thermal racemization of 1 is actually somewhat slower than that of 2, and that allylic rearrangements play no significant role in the racemization of 1. Optically active (+)-(R)-1, prepared by hexachlor odisilane reduction of (+)-(R)-allymethylphenylphosphine oxide,5 undergoes racemization in benzene with first-order kinetics:6 k,,, X lo5 (sec-I) = 1.69 (130"), 7.48(145"), and 28.0 (160"), whence E, = (1) We are grateful to the Air Force Office of Scientific Research for support of this research under Grant No. AF-AFOSR-1188-B. (2) L. Horner, H. Winkler, A. Rapp, A. Mentrup, H. Hoffmann, and P. Beck, TetrahedronLett., 161 (1961). (3) For a related rearrangement, see T. J. Katz, C. R. Nicholson, and C. A. Reilly, J . Amer. Chem. Soc., 88, 3832 (1966). (4) K . Naumann, G. Zon, and I
